I'm relatively new to A/D converters and such, and was wondering if I can put two or more A/D converters on the same clock and/or data out lines, while leaving the CS pins on different lines? Or maybe sharing the CS and clock pins, leaving the data out lines separate?
My goal is to minimize pin use on the microcontroller.
Thanks in advance! Dave
If it's SPI, you can common the clock and data lines, and have separate CS- decodes. If you do hang a lot of loads on the clock, beware of ringing... terminate at the end of the run, or at least leave a hook. Use an external CS decoder (HC138, whatever) if you're really hard up for pins.
Or you could read them out simultaneously (into an FPGA, for example) by splitting only the data lines.
John
For least pin use it's generally much better to use a multi input A/D chip that has channel switching built in. Eg ... 8 channels, 12 bit, serial (cheap!), such as the Microchip type MCP3208. You've 8 seperate voltage inputs to play with but only need 4 micro' pins allocating to drive the chip. regards john
The CLK line is an output from your micro and an input to all the serial ADCs, so that can be wired in common. The DATA lines are outputs from the serial ADCs and that's a problem. You can either wire the DATA lines to separate pins on the micro or else check to see if the DATA line goes tristate if the CS line is inactive. If so, then wire them in common as well and just don't drive two CS lines active at once. The CS lines will need to be on separate lines to the micro. If you have more than a few ADCs you can use a binary decoder, like the
74LS138 or the 74LS154, to reduce micro pin count at the expense of another IC.Jon
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Yes, no problem. If you separate the data-out you can sample both ADC's at the very same moment, which may be an advantage. It could also be a bit faster, you only need to bitbang the clock once for two DAC's being sampled at the same time.
-- Thanks, Frank. (remove 'x' and 'invalid' when replying by email)
You presumably speak of converters with SPI interface. In this case, yes, you can share the clock and data pins. Just to be sure, check whether the converter tri-states the data pin while its CS line is inactive. If it is really SPI compatible, it should do this, but if not, you may have to add additional buffers.
You can also share CS and clock pins while keeping the data pin separate. In this case tri-stating the data pin isn't necessary. The downside is that you can only read all ADCs simultaneously.
If you use ADCs with a parallel output bus, you can also share the data lines if they are tri-stated while CS is inactive.
-- Cheers Stefan
Most multi-input ADCs use a common reference voltage, I believe. That may (or may not) be a problem. Also, even in the better cases (if memory serves) isolation is usually not better than 80db between the inputs and too often less than that. For 12-bit, it again may or may not be a problem. But for 16-bit it may be an issue.
Jon
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